Exhaust after-treatment in heavy-duty motor vehicles

ABSTRACT

Controlling exhaust after-treatment in a heavy-duty motor vehicle includes operating a diesel engine of a heavy-duty truck such that the diesel engine generates an exhaust gas flow that enters an exhaust after-treatment system of the heavy-duty truck, the exhaust after-treatment system including a selective catalytic reduction system, measuring a level of NOx gases in the exhaust gas flow downstream of the selective catalytic reduction system, and controlling a diesel exhaust fluid injector upstream of the selective catalytic reduction system to inject diesel exhaust fluid into the exhaust gas flow upstream of the selective catalytic reduction system at an injection rate that is based on the measured level of NOx gases.

BACKGROUND Technical Field

The present disclosure relates generally to control of selectivecatalytic reduction in heavy-duty motor vehicle engines and, morespecifically, to closed-loop control of DEF dosing in selectivecatalytic reduction systems in heavy-duty motor vehicle exhaustafter-treatment systems.

Description of the Related Art

Regulated emissions from today's heavy-duty engines demand very lowlevels of tailpipe emissions, and standards are expected to be furtherreduced in the near future. To reduce tailpipe exhaust emissions,current technologies rely on aggressive engine control strategies andexhaust after-treatment catalyst systems (catalyst systems used to treatengine exhaust are referred to herein as exhaust after-treatmentsystems, emissions after-treatment systems, or EAS). The EAS for atypical heavy-duty diesel or other lean-burning engine may include adiesel oxidation catalyst (DOC) to oxidize unburned fuel and carbonmonoxide, a diesel particulate filter (DPF) for control of particulatematter (PM), selective catalytic reduction (SCR) systems for reductionof oxides of nitrogen (NO_(x), including at least NO and NO₂), and/or anammonia oxidation catalyst (AMOX). Performance of EAS systems, and ofSCR systems in particular, is dependent upon exhaust gas temperature andother parameters.

SCR processes use catalysts to catalyze the NO_(x) reduction and a fluidreferred to as DEF (diesel exhaust fluid), which acts as a NO_(x)reductant over the SCR catalyst. DEF is an aqueous solution thatevaporates and decomposes to chemically release ammonia so that theammonia is available for reaction. Efficiency of SCR operation isdependent upon temperature. For example, DEF evaporation anddecomposition is dependent upon temperature, with higher temperaturesgenerally improving performance. Temperature levels required to ensurecompliance with emissions regulations may be highly dependent upon awide variety of variables and are in some cases determinedexperimentally for specific engines, trucks, and operating conditionsthereof. Thus, an EAS may include a heater to increase the temperatureof the exhaust, to facilitate DEF injection, evaporation, anddecomposition at rates sufficient to allow efficient performance of theSCR processes.

BRIEF SUMMARY

A method may be summarized as comprising: operating a diesel engine of aheavy-duty truck such that the diesel engine generates an exhaust gasflow that enters an exhaust after-treatment system of the heavy-dutytruck, the exhaust after-treatment system including a selectivecatalytic reduction system; measuring a level of NO_(x) gases in theexhaust gas flow downstream of the selective catalytic reduction system;and controlling a diesel exhaust fluid injector upstream of theselective catalytic reduction system to inject diesel exhaust fluid intothe exhaust gas flow upstream of the selective catalytic reductionsystem at an injection rate that is based on the measured level ofNO_(x) gases.

The selective catalytic reduction system may be an underbody selectivecatalytic reduction system or a close-coupled selective catalyticreduction system where the injection rate is further based on atemperature of an underbody selective catalytic reduction system.

The selective catalytic reduction system may be an upstream selectivecatalytic reduction system, the diesel exhaust fluid injector may be afirst diesel exhaust fluid injector, and the method may furthercomprise: measuring a level of NO_(x) gases in the exhaust gas flowdownstream of a downstream selective catalytic reduction system; andcontrolling a second diesel exhaust fluid injector downstream of theupstream selective catalytic reduction system and upstream of thedownstream selective catalytic reduction system to inject diesel exhaustfluid into the exhaust gas flow downstream of the upstream selectivecatalytic reduction system and upstream of the downstream selectivecatalytic reduction system at an injection rate that is based on themeasured level of NO_(x) gases in the exhaust gas flow downstream of thedownstream selective catalytic reduction system.

The method may further comprise: determining whether ammonia slip isoccurring at the selective catalytic reduction system; and adjusting theinjection rate based on whether ammonia slip is occurring at theselective catalytic reduction system. Determining whether ammonia slipis occurring may include: adjusting the injection rate, thereby changingan ammonia-to-NO_(x) ratio at the selective catalytic reduction system;measuring first levels of NO_(x) gases in the exhaust gas flow upstreamof the selective catalytic reduction system and second levels of NO_(x)gases in the exhaust gas flow downstream of the selective catalyticreduction system as the ammonia-to-NO_(x) ratio at the selectivecatalytic reduction system changes; determining a measured efficiency ofthe selective catalytic reduction system based on the measured first andsecond levels of NO_(x) gases; and determining whether the measuredefficiency is positively or negatively correlated with the changingammonia-to-NO_(x) ratio. The method may further comprise, if it isconcluded that ammonia slip is not occurring and the measured efficiencyis less than a target efficiency, then increasing the injection rate, ifit is concluded that ammonia slip is not occurring and the measuredefficiency is greater than the target efficiency, then decreasing theinjection rate, and, if it is concluded that ammonia slip is occurring,then decreasing the injection rate.

A method may be summarized as comprising: injecting diesel exhaust fluidinto an exhaust after-treatment system upstream of a selective catalyticreduction system at an injection rate; measuring a NO_(x) leveldownstream of the selective catalytic reduction system; and adjustingthe injection rate based on the measured NO_(x) level.

The selective catalytic reduction system may be an underbody selectivecatalytic reduction system or a close-coupled selective catalyticreduction system where the injection rate is further based on atemperature of an underbody selective catalytic reduction system.

The selective catalytic reduction system may be an upstream selectivecatalytic reduction system, the injection rate may be a first injectionrate, and the method may further comprise: injecting diesel exhaustfluid into the exhaust after-treatment system downstream of the upstreamselective catalytic reduction system and upstream of a downstreamselective catalytic reduction system at a second injection rate;measuring a NO_(x) level downstream of the downstream selectivecatalytic reduction system; and adjusting the second injection ratebased on the measured NO_(x) level downstream of the downstreamselective catalytic reduction system.

The method may further comprise: determining whether ammonia slip isoccurring at the selective catalytic reduction system; and adjusting theinjection rate based on whether ammonia slip is occurring at theselective catalytic reduction system. Determining whether ammonia slipis occurring may include: adjusting the injection rate, thereby changingan ammonia-to-NO_(x) ratio at the selective catalytic reduction system;measuring first levels of NO_(x) gases in the exhaust gas flow upstreamof the selective catalytic reduction system and second levels of NO_(x)gases in the exhaust gas flow downstream of the selective catalyticreduction system as the ammonia-to-NO_(x) ratio at the selectivecatalytic reduction system changes; determining a measured efficiency ofthe selective catalytic reduction system based on the measured first andsecond levels of NO_(x) gases; and determining whether the measuredefficiency is positively or negatively correlated with the changingammonia-to-NO_(x) ratio. The method may further comprise, if it isconcluded that ammonia slip is not occurring and the measured efficiencyis less than a target efficiency, then increasing the injection rate, ifit is concluded that ammonia slip is not occurring and the measuredefficiency is greater than the target efficiency, then decreasing theinjection rate, and, if it is concluded that ammonia slip is occurring,then decreasing the injection rate.

A heavy-duty truck may be summarized as comprising: a diesel engine; anexhaust after-treatment system having an upstream end and a downstreamend opposite the upstream end, the upstream end coupled to the dieselengine, the exhaust after-treatment system including a selectivecatalytic reduction system; and an electronic control unit configuredto: operate the diesel engine such that the diesel engine generates anexhaust gas flow that enters the exhaust after-treatment system; recorda measurement of a level of NO_(x) gases in the exhaust gas flowdownstream of the selective catalytic reduction system; and control adiesel exhaust fluid injector upstream of the selective catalyticreduction system to inject diesel exhaust fluid into the exhaust gasflow upstream of the selective catalytic reduction system at aninjection rate that is based on the measured level of NO_(x) gases.

The electronic control unit may be further configured to: determinewhether ammonia slip is occurring at the selective catalytic reductionsystem; and adjust the injection rate based on whether ammonia slip isoccurring at the selective catalytic reduction system. To determinewhether ammonia slip is occurring, the electronic control unit may befurther configured to: adjust the injection rate, thereby changing anammonia-to-NO_(x) ratio at the selective catalytic reduction system;record measurements of first levels of NO_(x) gases in the exhaust gasflow upstream of the selective catalytic reduction system and secondlevels of NO_(x) gases in the exhaust gas flow downstream of theselective catalytic reduction system as the ammonia-to-NO_(x) ratio atthe selective catalytic reduction system changes; determine a measuredefficiency of the selective catalytic reduction system based on themeasured first and second levels of NO_(x) gases; and determine whetherthe measured efficiency is positively or negatively correlated with thechanging ammonia-to-NO_(x) ratio. The electronic control unit may befurther configured to: if it is concluded that ammonia slip is notoccurring and the measured efficiency is less than a target efficiency,then increase the injection rate, if it is concluded that ammonia slipis not occurring and the measured efficiency is greater than the targetefficiency, then decrease the injection rate, and, if it is concludedthat ammonia slip is occurring, then decrease the injection rate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a diagram of an exhaust after-treatment systemincluding a DOC, a DPF, and dual SCR systems.

FIG. 2A is a chart illustrating expected and measured SCR efficiencies,as an ammonia-to-NO_(x) ratio changes over time, when ammonia slip doesnot occur.

FIG. 2B is a chart illustrating expected and measured SCR efficiencies,as an ammonia-to-NO_(x) ratio changes over time, when ammonia slip doesoccur.

FIG. 3 is a chart illustrating measured SCR efficiencies as anammonia-to-NO_(x) ratio changes, when ammonia slip does and does notoccur.

FIG. 4 is a chart illustrating measured SCR efficiency as anammonia-to-NO_(x) ratio changes, when ammonia slip does and does notoccur.

FIG. 5 is a diagram of techniques for controlling DEF dosing in an SCRsystem.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures associated with the technology have notbeen shown or described in detail to avoid unnecessarily obscuringdescriptions of the embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments. Also, as used in thisspecification and the appended claims, the singular forms “a,” “an,” and“the” include plural referents unless the context clearly dictatesotherwise. It should also be noted that the term “or” is generallyemployed in its sense including “and/or” unless the context clearlydictates otherwise.

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Terms of geometric alignment may be used herein. Any components of theembodiments that are illustrated, described, or claimed herein as beingaligned, arranged in the same direction, parallel, or having othersimilar geometric relationships with respect to one another have suchrelationships in the illustrated, described, or claimed embodiments. Inalternative embodiments, however, such components can have any of theother similar geometric properties described herein indicating alignmentwith respect to one another. Any components of the embodiments that areillustrated, described, or claimed herein as being not aligned, arrangedin different directions, not parallel, perpendicular, transverse, orhaving other similar geometric relationships with respect to oneanother, have such relationships in the illustrated, described, orclaimed embodiments. In alternative embodiments, however, suchcomponents can have any of the other similar geometric propertiesdescribed herein indicating non-alignment with respect to one another.

Various examples of suitable dimensions of components and othernumerical values may be provided herein. In the illustrated, described,and claimed embodiments, such dimensions are accurate to within standardmanufacturing tolerances unless stated otherwise. Such dimensions areexamples, however, and can be modified to produce variations of thecomponents and systems described herein. In various alternativeembodiments, such dimensions and any other specific numerical valuesprovided herein can be approximations wherein the actual numericalvalues can vary by up to 1, 2, 5, 10, 15 or more percent from thestated, approximate dimensions or other numerical values.

Traditionally, heavy-duty vehicles included many components of exhaustafter-treatment systems “underbody,” that is, underneath the engine,cab, or another portion of the vehicle, where space is relatively freelyavailable and these components can therefore generally be larger thanwould otherwise be practical. Some modern heavy-duty vehicles, however,have begun to include a “close-coupled,” “up-close,” or “light-off” SCRunit much closer to the engine and exhaust ports thereof (e.g., adjacentto a turbine outlet of a turbocharger) and upstream of the traditionalunderbody exhaust after-treatment system, which can provide certainadvantages in that the temperature of the engine exhaust may be higherwhen it is closer to the engine, although locating an SCR unit nearerthe engine limits the available space and thus its practical size. Thus,some modern heavy-duty vehicles have included both a “close-coupled” SCRunit upstream with respect to the flow of the exhaust, such as adjacentto a turbine outlet of a turbocharger, to take advantage of the higherexhaust temperatures, as well as an “underbody” SCR unit downstream withrespect to the flow of the exhaust, such as under the engine or cab ofthe vehicle, to take advantage of the greater available space.

FIG. 1 illustrates a diagram of an exhaust after-treatment system 100that has a first, upstream end 102 and a second, downstream end 104opposite to the first, upstream end 102. The exhaust after-treatmentsystem 100 is a component of a vehicle, such as a large, heavy-duty,diesel truck, and in use carries exhaust from the diesel engine of thetruck to a tailpipe of the truck. For example, the first, upstream end102 of the exhaust after-treatment system 100 may be coupled directly toan exhaust port or an outlet port of the diesel engine, such as aturbine outlet of a turbocharger thereof, and the second, downstream end104 may be coupled directly to an inlet port of a tailpipe or muffler ofthe truck. Thus, when the engine is running and generating exhaust, theexhaust travels along the length of the exhaust after-treatment system100 from the first, upstream end 102 thereof to the second, downstreamend 104 thereof.

As illustrated in FIG. 1 , the exhaust after-treatment system 100includes, at its first, upstream end 102, or proximate or adjacentthereto, a first temperature sensor 106, which may be a thermocouple, tomeasure the temperature of the exhaust gas flow as it leaves the engineand enters the exhaust after-treatment system 100, before heat begins tobe lost through the exhaust after-treatment system 100 to theenvironment. The exhaust after-treatment system 100 also includes, atits first, upstream end 102, or proximate or adjacent thereto, or justdownstream of the first temperature sensor 106, a first NO_(x) sensor108, to measure the content of NO_(x) gases in the exhaust gas flow asit leaves the engine and enters the exhaust after-treatment system 100.The exhaust after-treatment system 100 also includes, at its first,upstream end 102, or proximate or adjacent thereto, or just downstreamof the first NO_(x) sensor 108, a first DEF injector 110, to inject DEFinto the exhaust gas flow as it leaves the engine and enters the exhaustafter-treatment system 100.

The exhaust after-treatment system 100 may also include, proximate oradjacent its first, upstream end 102, or just downstream of the firstDEF injector 110, a first heater 112, which may be anelectrically-powered resistive heater or heating element, a burner, orany other suitable heater, to inject heat energy into the exhaust gasflow and the injected DEF as they flow through the exhaustafter-treatment system 100. The exhaust after-treatment system 100 alsoincludes, just downstream of the first heater 112, a second temperaturesensor 114, which may be a thermocouple, to measure the temperature ofthe exhaust gas flow as it leaves the first heater 112 and just beforeor just as it enters a first, close-coupled SCR system 116, or at theinlet to the close-coupled SCR system 116. The exhaust after-treatmentsystem 100 also includes, just downstream of the first heater 112 andthe second temperature sensor 114, the first, close-coupled SCR system116, which is configured to reduce oxides of nitrogen (NO_(x)) in theexhaust gas flow.

The exhaust after-treatment system 100 also includes, just downstream ofthe first SCR system 116, a third temperature sensor 136, which may be athermocouple, to measure the temperature of the exhaust gas flow as itleaves the first SCR system 116. In some implementations, the secondtemperature sensor 114 and the third temperature sensor 136 may becollectively referred to as an SCR bed temperature sensor. For example,a temperature of a catalytic bed of the first, close-coupled SCR system116 may be measured, calculated, estimated, or otherwise determinedbased on the measurements provided by the second temperature sensor 114and the third temperature sensor 136, such as by averaging thetemperature measurements provided by the second temperature sensor 114and the third temperature sensor 136.

The exhaust after-treatment system 100 also includes, just downstream ofthe first SCR system 116 and/or the third temperature sensor 136, asecond NO_(x) sensor 118, to measure the content of NO_(x) gases in theexhaust gas flow as it leaves the first SCR system 116. In practice, thefirst NO_(x) sensor 108 and the second NO_(x) sensor 118 can be usedtogether to monitor, assess, or measure the performance of the first SCRsystem 116. Together, the first temperature sensor 106, the first NO_(x)sensor 108, the first DEF injector 110, the first heater 112, the secondtemperature sensor 114, the first, close-coupled SCR system 116, thethird temperature sensor 136, and the second NO_(x) sensor 118 can bereferred to as a close-coupled portion of the exhaust after-treatmentsystem 100, as they can be collectively located at or adjacent to theengine of the vehicle.

The exhaust after-treatment system 100 also includes, downstream of thefirst SCR system 116, the third temperature sensor 136, and the secondNO_(x) sensor 118, a DOC component 120, to oxidize unburned fuel andcarbon monoxide in the exhaust gas flow. The exhaust after-treatmentsystem 100 also includes, downstream of the DOC component 120, a DPF122, to reduce or otherwise control particulate matter in the exhaustgas flow. The exhaust after-treatment system 100 also includes,downstream of the DPF 122, a fourth temperature sensor 130, which may bea thermocouple, to measure the temperature of the exhaust gas flow as itleaves the DPF 122. The exhaust after-treatment system 100 alsoincludes, downstream of the DPF 122, or just downstream of the fourthtemperature sensor 130, a second DEF injector 124, to inject DEF intothe exhaust gas flow as it leaves the DPF 122.

In some embodiments, the exhaust after-treatment system 100 may alsoinclude, just downstream of the fourth temperature sensor 130 and thesecond DEF injector 124, a mixer 132 and a second heater, which may bean electrically-powered resistive heater or heating element, a burner,or any other suitable heater, to inject heat energy into the exhaust gasflow and the injected DEF as they flow through the exhaustafter-treatment system 100. The exhaust after-treatment system 100 alsoincludes, just downstream of the mixer 132 and the second heater, afifth temperature sensor 134, which may be a thermocouple, to measurethe temperature of the exhaust gas flow as it leaves the second heaterand just before or just as it enters a second, underbody SCR system 126,or at the inlet to the underbody SCR system 126. The exhaustafter-treatment system 100 also includes, just downstream of the mixer132, the second heater, and the fifth temperature sensor 134, thesecond, underbody SCR system 126, which is configured to reduce oxidesof nitrogen (NO_(x)) in the exhaust gas flow.

The exhaust after-treatment system 100 also includes, just downstream ofthe second SCR system 126, a sixth temperature sensor 138, which may bea thermocouple, to measure the temperature of the exhaust gas flow as itleaves the second SCR system 126. In some implementations, the fifthtemperature sensor 134 and the sixth temperature sensor 138 may becollectively referred to as an SCR bed temperature sensor. For example,a temperature of a catalytic bed of the second, underbody SCR system 126may be measured, calculated, estimated, or otherwise determined based onthe measurements provided by the fifth temperature sensor 134 and thesixth temperature sensor 138, such as by averaging the temperaturemeasurements provided by the fifth temperature sensor 134 and the sixthtemperature sensor 138.

In some alternative embodiments, the exhaust after-treatment system 100may not include the second heater and may include only a single heater,i.e., the first heater 112, to reduce overall costs. Similarly, in someembodiments, the exhaust after-treatment system 100 may not include allof the temperature sensors described herein, such as the thirdtemperature sensor 136, fourth temperature sensor 130, fifth temperaturesensor 134, and/or sixth temperature sensor 138, such as to furtherreduce overall costs. In such implementations, such temperature sensorsmay be replaced by virtual temperature sensors, which may measure,calculate, estimate, simulate, or otherwise determine a temperature atthe same location, such as based on equations, data, simulations, and/ormodels of the behavior of temperatures at such locations under theoperating conditions of the systems described herein.

The exhaust after-treatment system 100 also includes, just downstream ofthe second SCR system 126 and/or the sixth temperature sensor 138, andat its second, downstream end 104, or proximate or adjacent thereto, athird NO_(x) sensor 128, to measure the content of NO_(x) gases in theexhaust gas flow as it leaves the second SCR system 126. In practice,the second NO_(x) sensor 118 and the third NO_(x) sensor 128 can be usedtogether to monitor, assess, or measure the performance of the secondSCR system 126. Together, the DOC component 120, the DPF 122, the secondDEF injector 124, the fourth temperature sensor 130, the mixer 132, thesecond heater, the fifth temperature sensor 134, the second SCR system126, the sixth temperature sensor 138, and the third NO_(x) sensor 128can be referred to as an underbody portion of the exhaustafter-treatment system 100, as they can be collectively locatedunderneath the engine, cab, or another portion of the vehicle.

In general, performance of an SCR system at a given time can be assessedin terms of its conversion efficiency, which may be referred to as“efficiency,” and which may be measured as a percentage by which the SCRsystem reduces the quantity of NO_(x) gases (e.g., as measured by NO_(x)sensors as described herein) in an exhaust gas flow. An efficiency of anSCR system at a given time may be calculated based on a firstmeasurement of the content of NO_(x) gases in the exhaust gas flow justupstream of, or at an inlet of, the SCR system, and a second measurementof the content of NO_(x) gases in the exhaust gas flow just downstreamof, or at an outlet of, the SCR system. In particular, an efficiency canbe calculated as the ratio of the difference between the firstmeasurement and the second measurement to the first measurement. Thus,in the system 100, the efficiency of the first, close-coupled SCR system116 can be calculated as the ratio of the difference between themeasurement provided by the second NO_(x) sensor 118 and the measurementprovided by the first NO_(x) sensor 108 to the measurement provided bythe first NO_(x) sensor 108. Similarly, in the system 100, theefficiency of the second, underbody SCR system 126 can be calculated asthe ratio of the difference between the measurement provided by thethird NO_(x) sensor 128 and the measurement provided by the secondNO_(x) sensor 118 to the measurement provided by the second NO_(x)sensor 118.

DEF is used to provide ammonia that reacts with NO_(x) gases in anexhaust gas flow within SCR systems to form water and nitrogen (N₂). Ingeneral, a NO_(x) sensor upstream of an SCR system can measure a NO_(x)concentration in the exhaust gas flow as it enters the SCR system, andsuch a measurement can be used to control the amount of DEF injectedinto the exhaust gas flow. In some cases, a rate at which DEF isinjected can be selected to achieve a target ammonia-to-NO_(x) ratio(ANR) within the SCR system. For example, the rate at which DEF isinjected into the exhaust gas flow can be controlled to optimizeperformance of the SCR system (e.g., to achieve a high, maximum, ornear-maximum SCR efficiency, which may be less than 100% for a varietyof reasons, including wear of components, and which may depend on SCRbed temperature). In particular, the rate at which DEF is injected intothe exhaust gas flow can be controlled to facilitate operation of theSCR system at an efficiency within a range having a lower bound of 85%,86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%and/or having an upper bound of 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%,94%, 95%, 96%, 97%, 98%, 99%, or 100%. In some particular embodiments,the rate at which DEF is injected into the exhaust gas flow can becontrolled such that the SCR system operates at an efficiency of between90% and 100%. In some implementations, as the measured NO_(x)concentration upstream of an SCR system increases, the rate at which DEFis injected is increased so that additional ammonia is available forreaction with the NO_(x) gases within the SCR system, and as themeasured NO_(x) concentration upstream of the SCR system decreases, therate at which DEF is injected is decreased to avoid wasting DEF.

If DEF is injected into the exhaust gas flow at an excessive rate, thenexcess, unreacted ammonia can pass through the SCR system and the restof the exhaust after-treatment system and into the environment as acomponent of the vehicle's emissions. This can be referred to as“ammonia slip,” and can be problematic for a variety of reasons,including because it represents a waste in resources, an unnecessaryexpense, and an additional component of tailpipe emissions, and becauseit can lead to formation of deposits in the exhaust after-treatmentsystem and promote corrosion. Thus, it is important and valuable to beable to calculate and control DEF injection rates precisely andaccurately. If the DEF injection rate is too low, then there may not besufficient ammonia available in the SCR system to react with the NO_(x)gases and bring NO_(x) levels down to regulated levels. On the otherhand, if the DEF injection rate is too high, then there may be excessammonia available in the SCR system, resulting in ammonia slip. In someprevious exhaust after-treatment systems, where a NO_(x) sensor upstreamof an SCR system measures the NO_(x) concentration in the exhaust gasflow as it enters the SCR system, and this measurement is used tocontrol the amount of DEF injected into the exhaust gas flow, actual DEFinjection rates have in practice been lower than optimal for achievinghigh, maximum, or near-maximum SCR efficiencies. This has been done, insome cases, because fluctuations in injector performance, sensorperformance, and/or SCR performance, among other factors, leads toinherent uncertainty regarding appropriate DEF injection rates, andbecause, in some cases, it is preferable to avoid ammonia slip even ifit reduces SCR efficiency to some degree. As one specific example, if atheoretical maximum efficiency of an SCR system having a given bedtemperature is 95%, then it may be optimal to inject DEF at a rate thatprovides an ANR expected to result in an efficiency of 95%, but inpractice, some previous exhaust after-treatment systems wouldintentionally inject DEF at a lower rate to avoid ammonia slip or toreduce the chance of ammonia slip occurring given the inherentuncertainties.

The exhaust after-treatment system 100 includes a first, close-coupledSCR system 116 and a second NO_(x) sensor 118 directly downstreamthereof to measure the content of NO_(x) gases in the exhaust gas flowas it leaves the first SCR system 116, as well as a second, underbodySCR system 126 and a third NO_(x) sensor 128 directly downstream thereofto measure the content of NO_(x) gases in the exhaust gas flow as itleaves the second SCR system 126. Thus, measurements provided by thesecond and third NO_(x) sensors 118 and 128 can be used to calculatemeasured efficiencies in the SCR systems 116 and 126, which can be usedto provide closed-loop (and more precise) control of DEF dosing in theSCR systems 116 and 126, thereby preventing over-dosing and under-dosingof DEF. Of particular value is the ability to prevent over-dosing of DEFand resulting ammonia slip, which allows higher baseline levels of DEFdosing in normal operation, thereby improving overall SCR efficienciesand performance and facilitating efficiencies more closely approximatingtheoretical maximum efficiencies.

Implementing such closed-loop control of DEF dosing encountersdifficulties resulting from the fact that conventional NO_(x) sensors donot and/or cannot distinguish between NO_(x) and ammonia. For example, aconventional NO_(x) sensor may be an amperometric sensor with two orthree electrochemical cells adjacent to one another, where a first oneof the electrochemical cells pumps oxygen gas (O₂) out of the gas beingmeasured, and where a second one of the electrochemical cells performs aNO_(x) measurement on the gas being measured in the absence of theoxygen gas. Additional information regarding such sensors and theirsensitivity to ammonia can be found at:https://dieselnet.com/tech/sensors_nox.php andhttps://www.researchgate.net/publication/319069104_NOx_sensor_ammonia_cross_sensitivity_analysis_using_a_simplified_physics_based_model.Thus, if a conventional NO_(x) sensor is providing measurements justdownstream of an SCR system, e.g., at an outlet thereof, underconditions where ammonia slip is occurring, then the NO_(x) sensor willnot provide an accurate measurement of NO_(x) levels in the exhaust gasflow. In particular, and exacerbating this problem, such measurementswill indicate that NO_(x) levels are higher than is really the case,potentially triggering DEF injection rates to increase, leading toincreased ammonia slip and potentially causing a run-away cycle ofincreasing DEF injection. Specialized sensors that can measure ammoniaindependently of NO_(x) gases are available, and could be used toaddress this difficulty, but they are more expensive and complex than isdesired.

FIG. 2A illustrates expected and measured SCR efficiencies, aspercentages, as an ammonia-to-NO_(x) ratio (ANR) changes over time, whenammonia slip does not occur. In particular, FIG. 2A illustrates time inminutes on the x-axis (the ANR varies over time) and SCR efficiency inpercent on the y-axis, and plots an expected efficiency in a solid linenearer the top of the chart and an actual efficiency as measured byNO_(x) sensors at an inlet and an outlet of the SCR system in a brokenline nearer the bottom of the chart and below the plot of the expectedefficiency. This chart represents measurements under conditions whereammonia slip is not occurring, so the relationships between the expectedefficiency and the measured efficiency are straightforward and as onemight expect. They also illustrate the utility of the measurement of theactual efficiency, because it differs from (is less than) the expectedefficiency. Thus, faced with the results plotted in FIG. 2A, an exhaustafter-treatment system might increase an injection rate of DEF upstreamof the SCR system, such as until the plot of the measured efficiencycoincides with the plot of the expected efficiency or with a desired ortarget efficiency.

FIG. 2B illustrates expected and measured SCR efficiencies, aspercentages, as an ammonia-to-NO_(x) ratio (ANR) changes over time, whenammonia slip does occur. In particular, FIG. 2B illustrates time inminutes on the x-axis (the ANR varies over time) and SCR efficiency inpercent on the y-axis, and plots an expected efficiency in a solid linenearer the top of the chart and an actual efficiency as measured byNO_(x) sensors at an inlet and an outlet of the SCR system in a brokenline nearer the bottom of the chart and below the plot of the expectedefficiency. This chart represents measurements under conditions whereammonia slip is occurring, so the relationships between the expectedefficiency and the measured efficiency are not straightforward and notas one might expect—in fact, they are opposite one another, or mirrorimages across a horizontal line. They also illustrate, particularly incombination with the information illustrated in FIG. 2A, the difficultyin using conventional NO_(x) sensors to assess SCR efficiency, because,without knowing whether ammonia slip is occurring, efficiencies measuredby conventional NO_(x) sensors can be unreliable.

FIG. 3 illustrates measured SCR efficiencies, as percentages, as an ANRchanges, both when ammonia slip does occur and when ammonia slip doesnot occur. In particular, FIG. 3 illustrates ANR on the x-axis and SCRefficiency in percent on the y-axis, and plots a first measuredefficiency, in a solid line extending from the lower left to the upperright and having a positive slope, when ammonia slip is not occurring,and a second measured efficiency, in a broken line extending from theupper left to the lower right and having a negative slope, when ammoniaslip is occurring. FIG. 3 reveals a straightforward way to useconventional NO_(x) sensors, such as those in the exhaustafter-treatment system 100, to determine whether ammonia slip isoccurring. In particular, if a DEF injector is controlled to vary an ANRin an SCR system and the slope of the plot of the resulting SCRefficiency with ANR is positive (that is, if SCR efficiency ispositively correlated with ANR), then it can be concluded that ammoniaslip is not occurring, and if a DEF injector is controlled to vary anANR in an SCR system and the slope of the plot of the resulting SCRefficiency with ANR is negative (that is, if SCR efficiency isnegatively correlated with ANR), then it can be concluded that ammoniaslip is occurring.

FIG. 4 illustrates measured SCR efficiencies, as percentages, as an ANRchanges, both when ammonia slip does occur and when ammonia slip doesnot occur. In particular, FIG. 4 illustrates ANR on the x-axis and SCRefficiency in percent on the y-axis, and plots a measured efficiencywhile it has a positive slope, extending from the bottom left to the topcenter, while ammonia slip is not occurring, and while it has a negativeslope, extending from the top center to the bottom right, while ammoniaslip is occurring. FIG. 4 illustrates that there is an optimal or targetANR at the transition between the positively-sloped portion of the plotand the negatively-sloped portion of the plot. When it is determinedthat ANR is less than such a target ANR, it may be concluded thatefficiency is lower than optimal and a rate of DEF injection may beincreased as a result, and when it is determined that ANR is greaterthan such a target ANR, it may be concluded that ammonia slip isoccurring and a rate of DEF injection may be decreased as a result.

FIG. 5 is a diagram of techniques for controlling DEF dosing in an SCRsystem. For example, as illustrated at 150, a measurement T_(in) of atemperature at an inlet to the SCR system (e.g., as measured by thesecond temperature sensor 114 for first SCR system 116 or by the fifthtemperature sensor 134 for second SCR system 126) and a measurementT_(out) of a temperature at an outlet to the SCR system (e.g., asmeasured by the third temperature sensor 136 for first SCR system 116 orby the sixth temperature sensor 138 for second SCR system 126) can beused as inputs to a “target ANR table” at 152. The target ANR table maybe a lookup table unique to individual vehicle models and may have beencompiled based on experiments run on each of the specific vehiclemodels. In some cases the measurements T_(in) and T_(out) are useddirectly as inputs to the table at 152, while in other cases thesemeasurements can be used to calculate or estimate a temperature of acatalytic bed of the respective SCR system, and such a calculated orestimated temperature is then used directly as an input to the table at152. In either case, the input(s) are used to look up a target ANR forthe SCR system in the table at 152. At 154, a perturbation factor may beapplied to the target ANR (e.g., the target ANR may be multiplied by theperturbation factor). For example, in some embodiments, the perturbationfactor may be applied to the target ANR periodically to vary the targetANR, such that the target ANR increases slightly or decreases slightlyover short periods of time. In some cases, such variation can occurregularly, such as every 2, 3, 4, 5, 6, 8, 10, or 15 seconds. In someimplementations, the perturbation factor is above 1.0, such as 1.01, fora first period of time, and below 1.0, such as 0.99, for a second periodof time, where the first and second periods of time alternate with oneanother. In some specific implementations, the first period of time maybe the same as the second period of time, and either the first and/orthe second period of time can be greater than 1 second and less than 10seconds, such as 2, 3, 4, 5, 6, 7, 8, or 9 seconds.

As illustrated at 156, a measurement NO_(x_in) of a NO_(x) level at aninlet to the SCR system (e.g., as measured by the first NO_(x) sensor108 for first SCR system 116 or by the second NO_(x) sensor 118 forsecond SCR system 126, and which measurement may be taken over the lastor trailing second of a first or second period of time as discussed withrespect to the perturbation factor) and a measurement NO_(x_out) of aNO_(x) level at an outlet to the SCR system (e.g., as measured by thesecond NO_(x) sensor 118 for first SCR system 116 or by the third NO_(x)sensor 128 for second SCR system 126, and which measurement may be takenover the last or trailing second of a first or second period of time asdiscussed with respect to the perturbation factor) can be used as inputsto determine a “closed-loop correction factor” at 158. In some cases themeasurements NO_(x_in), and NO_(x_out), and/or the current ANR (e.g., anactual or measured ANR) can be used directly as inputs to thedetermination of the closed-loop correction factor at 158. In othercases the measurements NO_(x_in) and NO_(x_out) can be used to calculateor estimate an SCR system efficiency, and the calculated efficiencyand/or the actual or measured ANR can be used directly as inputs to thedetermination of the closed-loop correction factor at 158.

In other cases time-series data collected for the calculated efficiencyand the actual or measured ANR as the perturbation factor was used tovary the target ANR can be used to determine whether ammonia slip isoccurring. For example, if it is determined based on the time seriesdata that the calculated efficiency is positively correlated with theactual or measured ANR, then it can be concluded that ammonia slip isnot occurring, and if it is determined based on the time series datathat the calculated efficiency is negatively correlated with the actualor measured ANR, then it can be concluded that ammonia slip isoccurring. If it is concluded that ammonia slip is not occurring and itis determined that an efficiency of the SCR system calculated based onthe NO_(x_in) and NO_(x_out) measurements is less than a targetefficiency, then the closed-loop correction factor can be used todetermine an adjusted target ANR that is greater than the initial targetANR (e.g., the adjusted target ANR can be calculated by multiplying theinitial target ANR by a closed-loop correction factor that is greaterthan 1, or the adjusted target ANR can be calculated by adding aclosed-loop correction factor that is greater than 1 to the initialtarget ANR). If it is concluded that ammonia slip is not occurring andit is determined that an efficiency of the SCR system calculated basedon the NO_(x_in) and NO_(x_out) measurements is greater than a targetefficiency, then the closed-loop correction factor can be used todetermine an adjusted target ANR that is less than the initial targetANR (e.g., the adjusted target ANR can be calculated by multiplying theinitial target ANR by a closed-loop correction factor that is less than1 or the adjusted target ANR can be calculated by adding a closed-loopcorrection factor that is less than 1 to the initial target ANR). If itis concluded that ammonia slip is occurring, then the closed-loopcorrection factor can be used to determine an adjusted target ANR thatis less than the initial target ANR (e.g., the adjusted target ANR canbe calculated by multiplying the initial target ANR by a closed-loopcorrection factor that is less than 1 or the adjusted target ANR can becalculated by adding a closed-loop correction factor that is less than 1to the initial target ANR).

In some specific implementations, the closed-loop correction factor canbe initially set to 1.00, and can be updated based on measurements madeover time. For example, SCR bed temperatures can be periodicallycalculated as the average (e.g., mean) of measured temperatures at theinlet and the outlet of the SCR. Then, target efficiency can beperiodically determined, such as by reference to a lookup table, basedon the SCR bed temperature and the target ANR determined at 152. Next,measured efficiencies can be periodically calculated as the ratio of thedifference between a measurement NO_(x_in) of a NO_(x) level at an inletto the SCR system and a measurement NO_(x_out) of a NO_(x) level at anoutlet to the SCR system to the measurement NO_(x_in) of the NO_(x)level at the inlet to the SCR system, where such measurements are takenat the trailing end (e.g., trailing one second) of the time periodsduring which the perturbation factor is held constant. Efficiency errorscan then be periodically calculated by subtracting the calculatedmeasured efficiency from the determined target efficiency. In someimplementations, an efficiency slope can be calculated by performing theprevious steps repeatedly on time-series input data and then calculatinga ratio of a first measured efficiency subtracted from a second,subsequent measured efficiency to a first target efficiency subtractedfrom a second, subsequent target efficiency. In other implementations,an efficiency slope can be calculated by performing the previous stepsrepeatedly on time-series input data and then calculating a ratio of afirst measured efficiency subtracted from a second, subsequent measuredefficiency to a first target ANR subtracted from a second, subsequenttarget ANR.

In either case, once the efficiency slope has been calculated, and ifthe calculated efficiency slope is greater than an upper thresholdnumber, then the closed-loop correction factor can be increased, such asby an amount proportional to the efficiency error. In some alternativeimplementations, the closed-loop correction factor may be increased by afixed, constant value, rather than by an amount proportional to theefficiency error. Additionally, in either case, once the efficiencyslope has been calculated, and if the calculated efficiency slope isless than a lower threshold number, then the closed-loop correctionfactor can be decreased, such as by a fixed value. In some cases, such afixed value may be greater than 0.00, 0.01, 0.02, 0.03, or 0.04, and/orless than 0.01, 0.02, 0.03, 0.04, or 0.05. In some alternativeimplementations, the closed-loop correction factor may be decreased byan amount proportional to the efficiency error rather than a fixed,constant value. Further, in either case, once the efficiency slope hasbeen calculated, and if the calculated efficiency slope is less than theupper threshold number and greater than the lower threshold number, thenthe closed-loop correction factor can remain unchanged. In someimplementations, the upper threshold number may be greater than 0.0,0.1, 0.2, 0.3, or 0.4, and/or may be less than 0.6, 0.7, 0.8, 0.9, or1.0. In some implementations, the lower threshold number may be lessthan 0.0, −0.1, −0.2, −0.3, or −0.4, and/or may be greater than −0.6,−0.7, −0.8, −0.9, or −1.0. In some cases, the upper threshold number maybe opposite the lower threshold number.

Once a target ANR has been determined or an initial target ANR and anadjusted target ANR have been determined, such targets can be used tocalculate a base DEF dosing level at 160. For example, the calculatedDEF dosing level can be calculated to achieve the target ANR or adjustedtarget ANR in the SCR system, for example, based on any of the datadiscussed herein, including the measurement NO_(x_in) of a NO_(x) levelat an inlet to the SCR system, as well as a mass flow rate of theexhaust gas flow, M_(exh) For example, a DEF volumetric flow rate to beinjected into the exhaust gas flow can be calculated as the product ofthe exhaust mass flow rate, the molar ratio of NO_(x) in the exhaust gasflow (which can be determined directly from the measurement of theNO_(x) levels entering the SCR system), the target ANR or adjustedtarget ANR, and the molar mass of urea, divided by the molar mass of theexhaust, an ammonia to urea mole ratio (e.g., 2.0), the mass fraction ofurea in DEF, and the density of the DEF.

Once a base DEF dosing level has been calculated at 160, DEF dosinglimits can be applied at 162. For example, DEF dosing levels can belimited by minimum and maximum injection rates of the DEF injectionhardware used. Once the DEF dosing limits have been applied, a DEFdosing command can be transmitted to a DEF injector, such as one of theDEF injectors 110 and 124. As illustrated at 164, some threshold DEFdosing checks may be performed, such as based on the T_(in) and T_(out)inputs. For example, in some embodiments, a check may be performed toensure that an engine of the vehicle is running before any of themethods discussed herein are performed. As another example, the T_(in)and T_(out) inputs may be used to confirm that an SCR bed temperature ishigh enough to enable the SCR system to perform as intended before anyof the methods discussed herein are performed.

FIG. 5 and all of the features described thus far for FIG. 5 may beapplied to either a close-coupled SCR system, such as the first,close-coupled SCR system 116, and/or an underbody SCR system, such asthe second, underbody SCR system 126. In the case of such features beingapplied to a close-coupled SCR system, such as the first, close-coupledSCR system 116, as illustrated at 166, a measured temperature of thecatalytic bed of an underbody SCR system, such as the second, underbodySCR system 126, which may be determined based on temperaturemeasurements provided by the fifth temperature sensor 134 and/or thesixth temperature sensor 138, may be used as an input to a “target ANRmultiplicative factor table” at 166. The target ANR multiplicativefactor table may be a lookup table unique to individual vehicle modelsand may have been compiled based on experiments run on each of thespecific vehicle models. A multiplicative factor returned by the tablebased on the measured temperature of the underbody SCR system may beused to further adjust the target ANR (e.g., the target ANR may bemultiplied by the multiplicative factor), such that performance of theclose-coupled SCR system can be dependent upon temperatures at theunderbody SCR system.

In practice, a close-coupled SCR system may have a certain efficiency ata given time, which may be controlled to optimize overall performance ofthe entire exhaust after-treatment system, and, whatever the efficiencyof the close-coupled SCR system, the underbody SCR system may be used toreduce NO_(x) levels to within levels in compliance with applicableemissions regulations. Thus, to the extent the perturbation factorapplied at 154 affects efficiency of the close-coupled SCR system, theunderbody SCR system can make up the difference. To the extent theperturbation factor applied at 154 affects efficiency of the underbodySCR system, on the other hand, the baseline efficiency of the underbodySCR system can be increased to compensate. It has been found that theseeffects can be quite small. For example, for an SCR system operating ata baseline efficiency of 95%, it has been found that application of theperturbation factor at 154 as described herein can lead to variation ofthe baseline efficiency from a low of 94% to a high of 96%. That is, theeffect can be limited to within one percentage point in the efficiency,or about one percent of the baseline efficiency.

The exhaust after-treatment system 100 described herein includes thefirst, close-coupled SCR system 116 and the second, underbody SCR system126. In general terms, the exhaust after-treatment system 100 has afirst, upstream SCR system 116 and a second, downstream SCR system 126,where the upstream SCR system 116 is upstream of the downstream SCRsystem 126 and the downstream SCR system 126 is downstream of theupstream SCR system 116 with respect to the flow of exhaust through theexhaust after-treatment system 100.

In alternatives to the specific arrangement described with respect toFIG. 1 , an exhaust after-treatment system can have all of the featuresof the exhaust after-treatment system 100, and perform in accordancewith all of the methods described herein, except that both the upstreamand the downstream SCR systems are close-coupled SCR systems. In suchembodiments, the upstream SCR system may be referred to simply as a“first” SCR system or as a “first-in-box” SCR system. In suchembodiments, a first temperature sensor, a first NO_(x) sensor, a firstDEF injector, a first heater, a second temperature sensor, afirst-in-box SCR system, a third temperature sensor, and a second NO_(x)sensor can be referred to as a “first-in-box” portion of the exhaustafter-treatment system, as they can be collectively located relativelynear to the engine of the vehicle. Further, in such embodiments, a DOCcomponent, a DPF, a second DEF injector, a fourth temperature sensor, amixer, a second heater, a fifth temperature sensor, a “second-in-box”SCR system, a sixth temperature sensor, and a third NO_(x) sensor can bereferred to as a “second-in-box” portion of the exhaust after-treatmentsystem, as they can be collectively located relatively far from theengine of the vehicle.

In further alternatives to the specific arrangement described withrespect to FIG. 1 , an exhaust after-treatment system can have all ofthe features of the exhaust after-treatment system 100, and perform inaccordance with all of the methods described herein, except that boththe upstream and the downstream SCR systems are underbody SCR systems.In such embodiments, the upstream SCR system may be referred to simplyas a “first” SCR system or as a “first-in-box” SCR system. In suchembodiments, a first temperature sensor, a first NO_(x) sensor, a firstDEF injector, a first heater, a second temperature sensor, afirst-in-box SCR system, a third temperature sensor, and a second NO_(x)sensor can be referred to as a “first-in-box” portion of the exhaustafter-treatment system, as they can be collectively located relativelynear to the engine of the vehicle. Further, in such embodiments, a DOCcomponent, a DPF, a second DEF injector, a fourth temperature sensor, amixer, a second heater, a fifth temperature sensor, a “second-in-box”SCR system, a sixth temperature sensor, and a third NO_(x) sensor can bereferred to as a “second-in-box” portion of the exhaust after-treatmentsystem, as they can be collectively located relatively far from theengine of the vehicle.

The embodiments described herein provide closed-loop control of DEFdosing in SCR systems in exhaust after-treatment systems, particularlyin heavy-duty and/or diesel trucks. Such closed-loop control can reducevariability in NO_(x) levels throughout the system and improve operatingefficiencies of the SCR systems. Such closed-loop control further allowsoperating efficiencies to approach theoretical maximum efficiencieswhile avoiding ammonia slip, using traditional NO_(x) sensors, therebyeliminating or reducing the need for expensive components such as morespecialized sensors and/or ammonia oxidation catalyst systems. Suchclosed-loop control can be advantageous when regulated NO_(x) emissionlevels are relatively low or ultra-low.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

The invention claimed is:
 1. A method, comprising: operating a dieselengine of a heavy-duty truck such that the diesel engine generates anexhaust gas flow that enters an exhaust after-treatment system of theheavy-duty truck, the exhaust after-treatment system including aselective catalytic reduction system; measuring a level of NOx gases inthe exhaust gas flow downstream of the selective catalytic reductionsystem; controlling a diesel exhaust fluid injector upstream of theselective catalytic reduction system to inject diesel exhaust fluid intothe exhaust gas flow upstream of the selective catalytic reductionsystem at an injection rate that is based on the measured level of NOxgases; determining whether ammonia slip is occurring at the selectivecatalytic reduction system, including: adjusting the injection rate,thereby changing an ammonia-to-NOx ratio at the selective catalyticreduction system; measuring first levels of NOx gases in the exhaust gasflow upstream of the selective catalytic reduction system and secondlevels of NOx gases in the exhaust gas flow downstream of the selectivecatalytic reduction system as the ammonia-to-NOx ratio at the selectivecatalytic reduction system changes; determining a measured efficiency ofthe selective catalytic reduction system based on the measured first andsecond levels of NOx gases; and determining whether the measuredefficiency is positively or negatively correlated with the changingammonia-to-NOx ratio; and adjusting the injection rate based on whetherammonia slip is occurring at the selective catalytic reduction system.2. The method of claim 1 wherein the selective catalytic reductionsystem is an underbody selective catalytic reduction system.
 3. Themethod of claim 1 wherein the selective catalytic reduction system is aclose-coupled selective catalytic reduction system.
 4. The method ofclaim 3 wherein the injection rate is further based on a temperature ofan underbody selective catalytic reduction system.
 5. The method ofclaim 1 wherein the selective catalytic reduction system is an upstreamselective catalytic reduction system, the diesel exhaust fluid injectoris a first diesel exhaust fluid injector, and the method furthercomprises: measuring a level of NOx gases in the exhaust gas flowdownstream of a downstream selective catalytic reduction system; andcontrolling a second diesel exhaust fluid injector downstream of theupstream selective catalytic reduction system and upstream of thedownstream selective catalytic reduction system to inject diesel exhaustfluid into the exhaust gas flow downstream of the upstream selectivecatalytic reduction system and upstream of the downstream selectivecatalytic reduction system at an injection rate that is based on themeasured level of NOx gases in the exhaust gas flow downstream of thedownstream selective catalytic reduction system.
 6. The method of claim1, further comprising: if it is concluded that ammonia slip is notoccurring and the measured efficiency is less than a target efficiency,then increasing the injection rate; if it is concluded that ammonia slipis not occurring and the measured efficiency is greater than the targetefficiency, then decreasing the injection rate; and if it is concludedthat ammonia slip is occurring, then decreasing the injection rate.
 7. Amethod, comprising: injecting diesel exhaust fluid into an exhaustafter-treatment system upstream of an upstream selective catalyticreduction system at a first injection rate; measuring a first NOx leveldownstream of the upstream selective catalytic reduction system;adjusting the first injection rate based on the measured first NOxlevel; injecting diesel exhaust fluid into the exhaust after-treatmentsystem downstream of the upstream selective catalytic reduction systemand upstream of a downstream selective catalytic reduction system at asecond injection rate; measuring a second NOx level downstream of thedownstream selective catalytic reduction system; and adjusting thesecond injection rate based on the measured second NOx level downstreamof the downstream selective catalytic reduction system.
 8. A heavy-dutytruck, comprising: a diesel engine; an exhaust after-treatment systemhaving an upstream end and a downstream end opposite the upstream end,the upstream end coupled to the diesel engine, the exhaustafter-treatment system including a selective catalytic reduction system;and an electronic control unit configured to: operate the diesel enginesuch that the diesel engine generates an exhaust gas flow that entersthe exhaust after-treatment system; record a measurement of a level ofNOx gases in the exhaust gas flow downstream of the selective catalyticreduction system; control a diesel exhaust fluid injector upstream ofthe selective catalytic reduction system to inject diesel exhaust fluidinto the exhaust gas flow upstream of the selective catalytic reductionsystem at an injection rate that is based on the measured level of NOxgases; adjust the injection rate, thereby changing an ammonia-to-NOxratio at the selective catalytic reduction system; record measurementsof first levels of NOx gases in the exhaust gas flow upstream of theselective catalytic reduction system and second levels of NOx gases inthe exhaust gas flow downstream of the selective catalytic reductionsystem as the ammonia-to-NOx ratio at the selective catalytic reductionsystem changes; determine a measured efficiency of the selectivecatalytic reduction system based on the measured first and second levelsof NOx gases; determine whether the measured efficiency is positively ornegatively correlated with the changing ammonia-to-NOx ratio todetermine whether ammonia slip is occurring at the selective catalyticreduction system; and adjust the injection rate based on whether ammoniaslip is occurring at the selective catalytic reduction system.
 9. Theheavy-duty truck of claim 8, wherein the electronic control unit isfurther configured to: if it is concluded that ammonia slip is notoccurring and the measured efficiency is less than a target efficiency,then increase the injection rate; if it is concluded that ammonia slipis not occurring and the measured efficiency is greater than the targetefficiency, then decrease the injection rate; and if it is concludedthat ammonia slip is occurring, then decrease the injection rate.